Liquid electrostatic coating composition comprising corrosion resistant metal particulates and method for using same

BACKGROUND OF THE INVENTION

p-0002This invention broadly relates to a liquid electrostatic coating composition comprising a corrosion resistant metal particulate component having aluminum-containing metal particulates with a phosphate and/or silica-containing insulating layer; a glass-forming binder component; and a liquid carrier component. This invention also broadly relates to a method for using this liquid coating composition to electrostatically coat the metal substrate of an article.

p-0003A number of corrosion resistant coatings have been suggested for protecting the metal substrate of gas turbine components, as well as other articles requiring corrosion protection. Some of these corrosion resistant coatings are applied to or deposited on the metal substrate as sacrificial coatings. These coatings are “sacrificial” in that they preferentially react with and are used up by the corrodants, thus protecting the underlying metal substrate from attack by these corrodants.

p-0004Sacrificial corrosion resistant coatings for metal substrates can be formed from chromium or more typically aluminum, or from the respective oxides (i.e., alumina or chromia), by diffusion processes or techniques such as chemical vapor deposition or pack cementation. See, for example, commonly assigned U.S. Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994 (aluminide diffusion coating); commonly assigned U.S. Pat. No. 6,283,715 (Nagaraj et al), issued Sep. 4, 2001 (chromium diffusion coating); commonly assigned U.S. patent application Ser. No. 2004/0013802 A1 (Ackerman et al), published Jan. 22, 2004 (metal-organic chemical vapor deposition of aluminum, silicon, tantalum, titanium or chromium oxide on turbine disks and seal elements to provide a protective coating). Diffusion coating processes and techniques such chemical vapor deposition can be complicated and expensive processes for applying sacrificial corrosion resistant coatings to the metal substrate. These diffusion coatings also require some of the metal in the underlying substrate to be able to diffuse therefrom to form the coating.

p-0005These sacrificial coatings can also be applied to the metal substrate as aqueous compositions comprising phosphate binder systems and aluminum/alumina particles. See, for example, U.S. Pat. No. 4,606,967 (Mosser), issued Aug. 19, 1986 (spheroidal aluminum particles); U.S. Pat. No. 4,544,408 (Mosser et al), issued Oct. 1, 1985 (dispersible hydrated alumina particles). The phosphate-containing binder systems of these aqueous coating compositions typically comprise other binder materials, including chromates. See, for example, U.S. Pat. No. 3,248,249 (Collins, Jr.), issued Apr. 26, 1966; U.S. Pat. No. 3,248,251 (Allen), issued Apr. 26, 1966; U.S. Pat. No. 4,889,858 (Mosser), issued Dec. 26, 1989; U.S. Pat. No. 4,975,330 (Mosser), issued Dec. 4, 1990.

p-0006These aqueous coating compositions comprising phosphate-containing binders are typically applied by standard “wet spray” methods commonly used for spray painting. “Wet spray” methods are relatively easy and uncomplicated methods for applying these aqueous coating compositions to the surface of the metal substrate of the article to be coated. However, due to the lack of precision of “wet spray” methods for applying coatings and to ensure adequate coverage of the surface of the metal substrate to be coated, a significant amount of the aqueous coating composition does not end up on the metal substrate, but is instead lost due to “overspraying” thereof. Because of environmental concerns that can be created by such “overspraying,” aqueous coating compositions comprising phosphate-containing binders that are substantially free of chromates have been developed. See, for example, U.S. Pat. No. 6,368,394 (Hughes et al), issued Apr. 9, 2002 (substantially chromate free phosphate binder component). Even so, the lack of precision of “wet spray” application methods still makes it difficult to adequately and uniformly apply the coating composition to the surface of the metal substrate, especially a vertical extending surface, without significant and wasteful “overspraying” thereof.

p-0007Accordingly, there is still a need for sacrificial coating compositions that: (1) provide corrosion resistance for the underlying metal substrate; (2) can be applied to the metal substrate by relatively uncomplicated and inexpensive methods; (3) can be applied so as to adequately and uniformly cover the surface of the metal substrate, especially when that surface is vertical; (4) reduce or minimize wasteful “overspraying;” and (5) do not affect the intrinsic properties of the base metal or have a large interaction zone at the coating-metal interface.

BRIEF DESCRIPTION OF THE INVENTION

p-0008An embodiment of this invention broadly relates to a composition comprising a liquid mixture having: <ul><li id="ul0001-0001" num="0000"><ul><li id="ul0002-0001" num="0008">a corrosion resistant metal particulate component comprising aluminum-containing metal particulates, wherein the aluminum-containing metal particulates have a phosphate and/or silica-containing insulating layer;</li><li id="ul0002-0002" num="0009">a glass-forming binder component; and</li><li id="ul0002-0003" num="0010">a liquid carrier component.</li></ul></li></ul>

p-0009Another embodiment of this invention broadly relates to a method comprising the following steps: <ul><li id="ul0003-0001" num="0000"><ul><li id="ul0004-0001" num="0012">(a) providing an article comprising a metal substrate;</li><li id="ul0004-0002" num="0013">(b) imparting to the metal substrate an electrical charge; and</li><li id="ul0004-0003" num="0014">(c) electrostatically applying a liquid coating composition to the electrically charged metal substrate, wherein the liquid coating composition comprises a liquid mixture having: <ul><li id="ul0005-0001" num="0015">a corrosion resistant metal particulate component comprising aluminum-containing metal particulates, wherein the aluminum-containing metal particulates have a phosphate and/or silica-containing insulating layer;</li><li id="ul0005-0002" num="0016">a glass-forming binder component; and</li><li id="ul0005-0003" num="0017">a liquid carrier component.</li></ul></li></ul></li></ul>

p-0010The composition and method of this invention provide a number of significant benefits and advantages in providing sacrificial corrosion resistant coatings for articles comprising metal substrates. The liquid corrosion resistant metal coating composition can be applied by liquid electrostatic coating methods that are relatively uncomplicated and inexpensive. The embodiments of the liquid electrostatic coating method of this invention enable the corrosion resistant coating composition to be applied adequately and uniformly on the surface of the metal substrate to provide corrosion protection. The embodiments of the liquid electrostatic coating method of this invention can include electrostatic “wet spray” techniques similar to those used to apply other liquid coating compositions. The liquid corrosion resistant coating composition of this invention also does not adversely affect the intrinsic properties of the base metal of the substrate, or have a large interaction zone at the coating-metal interface.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0011<figref idrefs="DRAWINGS">FIG. 1</figref> is a perspective view of a turbine blade for which the coating composition of this invention is useful.

p-0012<figref idrefs="DRAWINGS">FIG. 2</figref> is a sectional view of the blade of <figref idrefs="DRAWINGS">FIG. 1</figref> with a corrosion resistant coating formed on the blade substrate using an embodiment of the coating composition of this invention.

p-0013<figref idrefs="DRAWINGS">FIG. 3</figref> is a sectional view similar to <figref idrefs="DRAWINGS">FIG. 2</figref> of a corrosion resistant coating comprising a plurality of layers formed by an embodiment of the method of this invention.

DETAILED DESCRIPTION OF THE INVENTION

p-0014As used herein, the term “particulate” refers to a particle, powder, flake, etc., that inherently exists in a relatively small form (e.g., a size of about 150 microns or less) or can be formed by, for example, grinding, shredding, fragmenting, pulverizing or otherwise subdividing a larger form of the material into a relatively small form.

p-0015As used herein, the term “corrosion resistant metal particulate component” refers to components of this invention comprising aluminum-containing metal particulates having a phosphate and/or silica-containing insulating layer, plus other optional corrosion resistant metal particulates. These other optional corrosion resistant metal particulates can comprise chromium, zirconium, nickel, cobalt, iron, titanium, yttrium, magnesium, hafnium, silicon, tantalum, etc., or combinations thereof. The particular level and amount of aluminum-containing metal particulates and optionally other metal particulates present in the corrosion resistant particulate component can be varied depending on the CTE properties desired for the resultant corrosion resistant coating, whether the corrosion resistant coating comprises a single layer or a plurality of layers, etc. Typically, the corrosion resistant metal particulate component comprises from about 1 to about 40% aluminum-containing metal particulates, more typically from about 2 to about 20% aluminum-containing metal particulates, with the balance being other (i.e., nonaluminum) metal particulates. The particulates comprising the corrosion resistant particulate component can have particle sizes in the range of from about 0.5 to about 150 microns, more typically in the range of from about 5 to about 40 microns, and can comprise particulates having unimodal, bimodal or polymodal particle size distributions.

p-0016As used herein, the term “aluminum-containing metal particulates” refers to metal particulates comprising aluminum, or aluminum alloys, i.e., aluminum alloyed with other metals. These other metals can include chromium, nickel, cobalt, iron, hafnium, zirconium, yttrium, tantalum, platinum, palladium, rhenium, silicon, etc., or combinations thereof. Suitable aluminum alloys include those having the formula: MAl, MCrAl, and MCrAlX, wherein M is nickel, cobalt, iron or a combination thereof, and X is hafnium, zirconium, yttrium, tantalum, platinum, palladium, rhenium, silicon or a combination thereof.

p-0017As used herein, the term “unimodal particle size distribution” refers to a particle size distribution comprising one particle size fraction. When graphically plotted, a unimodal particle size distribution has essentially a single peak.

p-0018As used herein, the term “bimodal particle size distribution” refers to a particle size distribution that comprises a smaller particle size fraction and a larger particle size fraction. When graphically plotted, a bimodal particle size distribution has essentially two distinct peaks.

p-0019As used herein, the term “polymodal particle size distribution” refers to a particle size distribution that comprises three or more particle size fractions. When graphically plotted, a polymodal particle size distribution has three or more distinct peaks.

p-0020As used herein, the term “phosphate and/or silica-containing insulating layer” refers to the electrostatically insulating layer comprising an electrically nonconductive phosphate, an electrically nonconductive silica or an electrically nonconductive phosphate and silica combination that surrounds, encloses, encapsulates, or otherwise coats, the aluminum-containing metal particulates (as well as any other metal particulates optionally present in the corrosion resistant metal particulate component) so that the corrosion resistant metal particulate component can be applied electrostatically to the surface of the metal substrate of the article. Suitable insulating layers can be derived from triethyl phosphate, ammonium monohydrogen phosphate, phosphorous pentoxide/phosphoric anhydride (typically dissolved in a solvent such as ethanol), tetraethyl orthosilicate, etc., or combinations thereof. The insulating layer can have any suitable thickness such that the metal particulates in the coating composition can be applied or deposited by electrostatic application methods on the surface of the metal substrate of the article. Typically, the insulating layer has a thickness of from about 0.01 to about 2 microns, more typically from about 0.1 to about 1 microns. This insulating layer can be formed on the metal particulates by any suitable process, including vapor deposition, physical vapor deposition, chemical vapor deposition, atomic layer epitaxy, sol-gel processing, electroplating, electroless plating, passivation/pickling chemical treatments, aqueous or gas-phase chemical surface treatments, etc. A particularly suitable approach for forming the insulating layer on the metal particulates is by precipitation of the phosphate and/or silica (in the form of the respective silicate precursor) from solution, for example, by a sol-gel process. When formed by a sol-gel process, the phosphate and/or silica forming the insulating layer is controllably precipitated from a solution containing a precursor thereof. See commonly assigned U.S. Pat. No. 6,379,804 (Ackerman et al), issued Apr. 30, 2002 (herein incorporated by reference), which discloses a sol-gel process that can be adapted or modified to form the phosphate and/or silica-containing insulating layer on the aluminum-containing metal particulates of this invention.

p-0021As used herein, the term “substantially free” means the indicated compound, material, component, etc., is minimally present or not present at all, e.g., at a level of about 0.5% or less, more typically at a level of about 0.1% or less, unless otherwise specified.

p-0022A used herein, the term “glass-forming binder component” refers to component that, when cured, forms a glassy matrix to which the particulates in the particulate component are embedded in, are encapsulated in, are enclosed by, or otherwise adhered to. This binder component can comprise glass particulates (e.g., glass powder, glass frit), frits comprising other inorganic minerals, as well as phosphate binder compounds, compositions, etc., alone or in combination with other optional binder materials. These phosphate binders are typically in the form of the respective phosphate compounds/compositions, including orthophosphates, pyrophosphates, etc. These phosphate compounds/compositions can be monobasic, dibasic, tribasic or any combination thereof. The phosphate binders can comprise one or more metal phosphates, including aluminum phosphates, magnesium phosphates, chromium phosphates, zinc phosphates, iron phosphates, lithium phosphates, etc, or any combination thereof. Typically, the phosphate binder comprises an aluminum phosphate, a magnesium phosphate, a chromium phosphate or a combination thereof. Other binder materials that can be included with the phosphate binder include one or more chromates, molybdates, etc. See, for example, U.S. Pat. No. 3,248,249 (Collins, Jr.), issued Apr. 26, 1966; U.S. Pat. No. 3,248,251 (Allen), issued April 26, 196604/26/66; U.S. Pat. No. 4,889,858 (Mosser), issued Dec. 26, 1989; U.S. Pat. No. 4,975,330 (Mosser), issued Dec. 4, 1990. The binder component can also be substantially free of binder materials other than phosphate binders, e.g., a substantially chromate free phosphate-containing binder component. See, for example, U.S. Pat. No. 6,368,394 (Hughes et al), issued Apr. 9, 2002 (substantially chromate free phosphate binder component).

p-0023As used herein, the term “liquid carrier component” refers to any carrier component that is liquid at ambient temperatures and in which the corrosion resistant metal particulate component and glass-forming binder component is typically carried in, dispersed in, dissolved in, etc. Liquid carrier components include aqueous systems (e.g., comprising water), organic systems (e.g., comprising liquid organic materials or solvents) or any combination thereof. The liquid carrier component can be formulated to provide any desired degree of polarity, i.e., to form polar or non-polar systems. Increasing the degree of polarity of the liquid carrier component can make the resulting liquid coating composition more conductive in electrostatic application methods, while descreasing the degree of polarity of the liquid carrier component can make the resulting liquid coating composition less conductive in electrostatic application methods. Non-polar organic liquids for making the liquid coating composition less conductive include toluene, xylene, mono and dialkyl ethers of ethylene glycol (e.g., Cellosolves®) such as ethylene glycol monoethy ether (2-ethoxy ethanol, Cellosolve® solvent), ethylene glycol monobutylether (2-butoxyethanol, butyl Cellosolve® acetate), ethylene glycol monobutylether acetate (2-ethoxylethyl acetate, Cellosolve® acetate), etc., alkyl acetates such as ethyl acetate, butyl acetate, ethyl 3-ethoxy propionate, etc., mineral spirits, hexane, octane, cyclohexane, etc. Polar organic liquids for making the liquid coating composition more conductive include aqueous liquids (e.g., water or water mixed with miscible organic solvents), nitroparaffins (e.g., nitromethane, nitroethane, nitropropane, etc.), alcohols (e.g., ethanol, propanol, butanol, diacetone alcohol, etc.), lacquer thinner, ketones (e.g., acetone, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, etc), etc. Combinations of non-polar and polar liquids can also be employed as liquid carrier components. The liquid carrier component can be of any desired consistency, flowability, viscosity, etc., but is typically formulated to provide a relatively low viscosity, flowable consistency such that the liquid coating composition can be applied by standard liquid electrostatic coating techniques, e.g., by spraying with an electrostatic paint sprayer or gun. The liquid carrier component is also typically formulated so as to evaporate or be volatilized after application, and especially after the applied coating composition is cured.

p-0024As used herein, the term “corrosion resistant coating” refers to coatings of this invention that, after curing, comprise a glassy binder matrix having embedded therein, encapsulated therein, enclosed thereby, or otherwise adhered thereto, metal particulates from the corrosion resistant metal particulate component. Corrosion resistant coatings of this invention can provide resistance against corrosion caused by various corrodants, including metal (e.g., alkaline) sulfates, sulfites, chlorides, carbonates, oxides, and other corrodant salt deposits resulting from dirt, fly ash, concrete dust, sand, sea salt, etc.

p-0025As used herein, the term “corrosion resistant metal coating composition” refers to coating compositions comprising the corrosion resistant metal particulate component (typically in an amount of from about 5 to about 75%, more typically in an amount of from about 30 to about 60%, of the composition), the glass-forming binder component (typically in an amount of from about 10 to about 80%, more typically in an amount of from about 30 to about 60%, of the composition), with the liquid carrier component (plus any other optional components such as silicas, colorants, pigments, etc.) typically comprising the balance of the composition. The corrosion resistant metal coating compositions of this invention are typically formulated to have or provide a liquid, flowable consistency, e.g., are formulated as liquid, flowable mixtures, that can be deposited by liquid electrostatic application methods or techniques.

p-0026As used herein, the term “curing” refers to any treatment condition or combination of treatment conditions that causes melting of the binder component to form a glassy matrix after the corrosion resistant metal coating composition is applied electrostatically to the metal substrate, and thereby forms the corrosion resistant coating. Typically, curing occurs by heating the applied corrosion resistant metal coating composition at a temperature of at least about 1200° F. (649° C.), more typically at least about 1550° F. (843° C.).

p-0027As used herein, the term “article” refers to any article comprising a metal substrate and requiring corrosion protection, including a wide variety of turbine engine (e.g., gas turbine engine) parts and components operated at, or exposed to, high temperatures, especially higher temperatures that occur during normal engine operation. These turbine engine parts and components can include turbine disks and turbine shafts, turbine components comprising airfoils such as turbine blades and vanes, turbine shrouds, turbine nozzles, combustor components such as liners, deflectors and their respective dome assemblies, augmentor hardware of gas turbine engines, exhaust etc. The corrosion resistant coatings of this invention are particularly useful for articles comprising metal substrates in the form of turbine blades and vanes, and especially the airfoil portions of such blades and vanes. However, while the following discussion of articles of this invention will be with reference to turbine blades and turbine vanes, and especially the airfoil portions thereof, that comprise these blades and vanes, it should also be understood that the corrosion resistant coatings of this invention can be useful with other articles comprising metal substrates that require corrosion protection.

p-0028As used herein, the term “CTE” refers to the coefficient of thermal expansion of a material, and is referred to herein in units of 10⁻⁶/° F. For example, alumina which has a coefficient of thermal expansion of about 4 to 5×10⁻⁶/° F. at about 1200° F. (649° C.) is referred to herein as having a CTE of about 4 to 5.

p-0029As used herein, the term “comprising” means various particulates, materials, coatings, compositions, components, layers, steps, etc., can be conjointly employed in the present invention. Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of.”

p-0030All amounts, parts, ratios and percentages used herein are by weight unless otherwise specified.

p-0031Compositions comprising aluminum-containing metal particulates and glass-forming binder systems, with or without additional chromate binders or other binder materials, can be used to provide corrosion resistant coatings for various articles comprising metal substrates, such as turbine blades and vanes. It would also be desirable to be able to apply these coating compositions comprising these aluminum-containing metal particulates by liquid electrostatic application methods and techniques. Because liquid electrostatic droplets are charged, and are thus attracted to the electrically charged metal substrate, electrostatic methods and techniques provide a more precise (targeted) application of the coating composition to adequately and uniformly cover the surface thereof so as to reduce the amount of drift and “overspray.” Liquid electrostatic coating compositions also reduce the emission of volatile constituents because less overspray occurs in use due to the inherent transfer efficiency of liquid electrostatic systems. Because of the electrostatic attraction of the coating droplets to the electrically charged metal substrate, liquid electrostatic application methods can enable the application of the liquid coating composition around corners of the metal substrate.

p-0032However, it has been discovered that prior corrosion resistant coatings comprising aluminum-containing metal particulates cannot be applied (e.g., sprayed) electrostatically so as to adhere to a metal substrate that has been electrically charged. Because prior aluminum-containing metal particulates are electrically conductive, it has been found that these aluminum-containing metal particulates, when applied under electrostatic conditions, do not retain their electrostatic charge for an adequate period of time. As a result, the electrostatic droplets comprising the aluminum-containing metal particulates do not hold the electrostatic charge very well, thus causing the sprayed coating comprised of the metal particulates to be applied non-uniformly on the metal substrate. Instead, after the coating composition is sprayed, especially on a substantially vertical surface of an electrically charged metal substrate, the sprayed droplets comprising the aluminum-containing metal particulates tend to disassociate from the metal substrate, or the sprayed coating tends to sag.

p-0033The liquid corrosion resistant metal coating compositions of this invention solve this problem by providing the aluminum-containing metal particulates with a phosphate and/or silica-containing insulating layer. This insulating layer permits the normally electrically conductive aluminum-containing metal particulates in the liquid coating composition to be applied by conventional liquid electrostatic application methods and techniques. In particular, when the liquid coating composition is applied, e.g., sprayed, under electrostatic conditions, the aluminum-containing metal particulates having the phosphate and/or silica-containing insulating layer adhere to the surface of the electrically charged metal substrate, even when that surface is vertically extending, for an adequate period of time for the applied coating to be cured. Indeed, the liquid coating composition comprising these insulated aluminum-containing metal particulates can be applied so as to flow around corners of the electrically charged metal substrate (commonly referred to as “wrap around”), and thus adhere to the corner surfaces, as well as portions of the surfaces extending beyond the corners of the metal substrate. These liquid electrostatic coating compositions thus combine the desirable features of targeted application, with the ease of application that can be provided by a “wet spray” technique.

p-0034The various embodiments of this invention are further illustrated by reference to the drawings as described hereafter. Referring to the drawings, <figref idrefs="DRAWINGS">FIG. 1</figref> depicts a component article of a gas turbine engine such as a turbine blade or turbine vane, and in particular a turbine blade identified generally as <b>10</b>. (Turbine vanes have a similar appearance with respect to the pertinent portions.) Blade <b>10</b> generally includes an airfoil <b>12</b> against which hot combustion gases are directed during operation of the gas turbine engine, and whose surfaces are therefore at risk to attack by environmental corrodants such as sea salt. Airfoil <b>12</b> has a “high-pressure side” indicated as <b>14</b> that is concavely shaped; and a suction side indicated as <b>16</b> that is convexly shaped and is sometimes known as the “low-pressure side” or “back side.” In operation the hot combustion gas is directed against the high-pressure side <b>14</b>. Blade <b>10</b> is anchored to a turbine disk (not shown) with a dovetail <b>18</b> formed on the root section <b>20</b> of blade <b>10</b>. In some embodiments of blade <b>10</b>, a number of internal passages extend through the interior of airfoil <b>12</b>, ending in openings indicated as <b>22</b> in the surface of airfoil <b>12</b>. During operation, a flow of cooling air is directed through the internal passages (not shown) to cool or reduce the temperature of airfoil <b>12</b>

p-0035Referring to <figref idrefs="DRAWINGS">FIG. 2</figref>, the base material of airfoil <b>12</b> of blade <b>10</b> that serves as the metal substrate <b>60</b> having the corrosion resistant coating of this invention can comprise any of a variety of metals, or more typically metal alloys, including those based on nickel, cobalt and/or iron alloys. Substrate <b>60</b> typically comprises a superalloy based on nickel, cobalt and/or iron. Superalloys are generally described in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol. 12, pp. 417-479 (1980), and Vol. 15, pp. 787-800 (1981). Illustrative nickel-based superalloys are designated by the trade names Inconel®, Nimonic®, Rene® (e.g., Rene® 88, Rene® 104 alloys), and Udimet®.

p-0036Substrate <b>60</b> more typically comprises a nickel-based alloy, and particularly a nickel-based superalloy, that has more nickel than any other element. The nickel-based superalloy can be strengthened by the precipitation of gamma prime or a related phase. A nickel-based superalloy for which the ceramic corrosion resistant coating of this invention is particularly useful is available by the trade name René 88, which has a nominal composition, by weight of 13% cobalt, 16% chromium, 4% molybdenum, 3.7% titanium, 2.1% aluminum, 4% tungsten, 0.70% niobium, 0.015% boron, 0.03% zirconium, and 0.03 percent carbon, with the balance nickel and minor impurities.

p-0037Prior to forming the corrosion resistant coating <b>64</b> of this invention on the surface <b>62</b> of metal substrate <b>60</b>, surface <b>62</b> is typically pretreated mechanically, chemically or both to make the surface more receptive for coating <b>64</b>. Suitable pretreatment methods include grit blasting, with or without masking of surfaces that are not to be subjected to grit blasting (see U.S. Pat. No. 5,723,078 to Niagara et al, issued Mar. 3, 1998, especially col. 4, lines 46-66, which is incorporated by reference), micromachining, laser etching (see U.S. Pat. No. 5,723,078 to Nagaraj et al, issued Mar. 3, 1998, especially col. 4, line 67 to col. 5, line 3 and 14-17, which is incorporated by reference), treatment (such as by photolithography) with chemical etchants such as those containing hydrochloric acid, hydrofluoric acid, nitric acid, ammonium bifluorides and mixtures thereof, (see, for example, U.S. Pat. No. 5,723,078 to Nagaraj et al, issued Mar. 3, 1998, especially col. 5, lines 3-10; U.S. Pat. No. 4,563,239 to Adinolfi et al, issued Jan. 7, 1986, especially col. 2, line 67 to col. 3, line 7; U.S. Pat. No. 4,353,780 to Fishter et al, issued Oct. 12, 1982, especially col. 1, lines 50-58; and U.S. Pat. No. 4,411,730 to Fishter et al, issued Oct. 25, 1983, especially col. 2, lines 40-51, all of which are incorporated by reference), treatment with water under pressure (i.e., water jet treatment), with or without loading with abrasive particles, as well as various combinations of these methods. Typically, the surface <b>62</b> of metal substrate <b>60</b> is pretreated by grit blasting where surface <b>62</b> is subjected to the abrasive action of silicon carbide particles, steel particles, alumina particles or other types of abrasive particles. These particles used in grit blasting are typically alumina particles and typically have a particle size of from about 600 to about 35 mesh (from about 20 to about 500 micrometers), more typically from about 400 to about 270 mesh (from about 38 to about 53 micrometers).

p-0038The corrosion resistant coating <b>64</b> can be formed on metal substrate <b>60</b> by a method comprising the following steps: (a) imparting to the metal substrate an electrical charge; (b) electrostatically applying the liquid coating composition to the electrically charged metal substrate; and (c) curing the applied liquid coating composition while electrostatically charged at a curing temperature that causes the corrosion resistant particulate component (i.e., aluminum metal particulates, plus any other metal particulates) and glass-forming binder component to form a corrosion resistant coating <b>64</b> that comprises a glassy matrix of binder to which the particulates in the particulate component are embedded in, encapsulated in, enclosed by, or otherwise adhered to. The coating composition can be applied to substrate <b>60</b> by any manner of liquid electrostatic application, including pouring, flowing, dipping, spraying, rolling, etc., to provide an uncured layer of the liquid composition comprising the particulates and binder. For example, the coating composition can be applied to substrate <b>60</b> using a conventional liquid electrostatic paint sprayer or gun system such as a Nordson Kintex liquid electrostatic spray system. See also, for example, U.S. Pat. No. 6,622,937 (Schroeder et al), issued Sep. 23, 2003; U.S. Pat. No. 5,725,161 (Hartle), issued Mar. 10, 1998; U.S. Pat. No. 5,341,989 (Fulkerson et al), issued Aug. 30, 1994; and U.S. Pat. No. RE 31,867 (Kennon), issued Apr. 16, 1985, which disclose some illustrative electrostatic spray guns suitable for use herein.

p-0039This applied liquid coating composition layer is then cured, typically by heating at a temperature of at least about 1200° F. (649° C.), more typically at least about 1550° F. (843° C.), to form corrosion resistant coating <b>64</b>. Coating <b>64</b> can be formed up to a thickness of at least about 10 mils (254 microns), and typically has a thickness in the range of from about 0.5 to about 5 mils (from about 13 to about 127 microns), more typically from about 1 to about 2.5 mils (from about 25 to about 64 microns).

p-0040Coating <b>64</b> can be formed as a single layer, or can be formed as a plurality of layers. In forming a plurality of layers in coating <b>64</b>, each respective layer can be formed by depositing the coating composition and then curing the deposited composition, with the layers being built by depositing the coating composition on the underlying layer that was previously formed. The respective layers can have the same or differing thicknesses. The coating composition used in forming each of the respective layers can having the same or differing levels of particulate component and binder component. In addition, the level of aluminum-containing metal particulates in the particulate component of the corrosion resistant coating composition can be same or different in the respective deposited layers of coating <b>64</b>. Each layer deposited can be cured to the same or different degrees. If desired, an outer glassy layer can be formed on coating <b>64</b> that comprises depositing and curing a coating composition consisting essentially of the glass-forming binder component that is substantially free of the particulate component.

p-0041An embodiment of a corrosion resistant coating of this invention comprising a plurality of layers is shown in <figref idrefs="DRAWINGS">FIG. 3</figref> and is indicated generally as <b>364</b>. As shown in <figref idrefs="DRAWINGS">FIG. 3</figref>, coating <b>364</b> comprises an inner layer <b>368</b> that is adjacent to and overlaying metal substrate <b>60</b>. Inner layer <b>368</b> is relatively thick and typically has a thickness of from about 0.5 to about 5 mils (from about 13 to about 127 microns), more typically from about 1 to about 2.5 mils (from about 25 to about 64 microns). Typically, the inner layer <b>368</b> comprises from about 5 to about 75% metal particulates, more typically from about 30 to about 60% metal particulates.

p-0042Coating <b>364</b> also comprises an intermediate layer indicated generally as <b>372</b> adjacent to and overlaying inner layer <b>368</b>. Intermediate layer <b>372</b> is typically thinner, especially relative to inner layer <b>368</b>. Intermediate layer <b>372</b> typically has a thickness of from about 0.1 to about 2.5 mils (from about 3 to about 64 microns), more typically from about 0.5 to about 1.5 mils (from about 13 to about 38 microns). Intermediate layer <b>372</b> can also comprise an increased or decreased amount or level of metal particulates than that present in inner layer <b>368</b>. Typically, intermediate layer <b>172</b> can comprise from about 10 to about 70% metal particulates, more typically from about 20 to about 50% metal particulates.

p-0043As shown in <figref idrefs="DRAWINGS">FIG. 3</figref>, coating <b>364</b> can further comprise an outer layer indicated generally as <b>376</b> adjacent to and overlaying intermediate layer. (In the absence of layer <b>376</b>, layer <b>372</b> would become the outer layer of coating <b>364</b>.) This outer layer <b>376</b> can comprise a particulate component, but is typically substantially free of particulates. Typically, outer layer <b>376</b> consists essentially of the glass-forming binder component to form a relatively hard, glassy layer. Outer layer <b>376</b> is also typically the thinnest layer of coating <b>364</b>, especially when substantially free of particulates. Typically, outer layer <b>376</b> has a thickness of from about 0.5 to about 5 mils (from about 13 to about 127 microns), more typically from about 1 to about 3 mils (from about 25 to about 76 microns).

p-0044While the above embodiments have been described in the context of coating turbine blades and vanes, this invention can be used to form corrosion resistant coatings, as described above, on the surfaces of other turbine components or other articles. The corrosion resistant coatings of this invention can also be formed during original manufacture of the component (i.e., an OEM component), after the component has been in operation for a period of time, after other coatings have been removed from the component (e.g., a repair situation), while the component is assembled or after the component is disassembled, etc.

p-0045While specific embodiments of this invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of this invention as defined in the appended claims.